This application claims the priority of German Patent Application Serial No. 101 23 157.1, filed Apr. 27, 2001, the subject matter of which is incorporated herein by reference.
The present invention relates, in general, to a pneumatic valve, and more particularly to a pneumatic valve for use in a pneumatic lifting drive for controlling the volume flow of an operating gas under pressure.
Pneumatic lifting drives oftentimes have the problem that the pneumatic drives, connected to a supply source to feed gas under pressure, lift too quickly or lower too slowly, when the load is small, or lift too slowly or lower too quickly, when the load is high. The reason is the compressibility and thus the energy storing capability of the operating gas, e.g. air. In order to realize a substantially even movement velocity of the pneumatic drive despite changing loads, the provision of a pneumatic valve is necessary to maintain a constant pressure-independent volume flow to and from the pneumatic drive.
German utility model DE 298 02 606 A1 describes the use of such a pneumatic valve for controlling the volume flow of operating gas. The pneumatic valve has a gas inlet port, a gas outlet port, and throttle valves disposed between the inlet and outlet ports and activated by a control unit. When incorporated in a lifting drive, the activation of the pneumatic valve is realized in dependence on the load, with a sensor-activation unit disposed in the force flux. This conventional pneumatic valve suffers shortcomings because the pneumatic valve and its throttle valves have to be disposed in the force flux, resulting in a heavy overall structure because the structure must correspond to the mechanical load.
Section 5.4.3 of publication xe2x80x9cPneumatische Steuerungstechnik [Pneumatic Control Techniques], R. Haug, Teubner-Verlag, Stuttgart 1991, page 228, proposes to maintain a constant volume flow by means of a differential pressure regulator with gas pressure taps on opposite sides of a throttle valve or orifice. This configuration has the drawback that the volume flow can be controlled only in one direction and that the overall structure is rather complicated.
It would therefore be desirable and advantageous to provide an improved pneumatic valve which obviates prior art shortcomings and is able to maintain a substantially constant, pressure-independent volume flow and which is useful for operation in both flow directions, while yet being reliable in operation and simple in structure.
According to one aspect of the present invention, a pneumatic valve, in particular for use in a pneumatic lifting drive, for controlling a volume flow of an operating gas supplied under pressure, includes a valve body having a main connection line for linking first and second ports for entry and exit of the operating gas, and a cross bore extending across the valve body and intersecting the main connection line; an adjustable throttle including a control element received in the cross bore for movement between first and second end positions and having a control opening for the operating gas with an effective area which progressively decreases in a direction to the second end position, as the control element moves from the first end position toward the second end position, and a counterforce member opposing a movement of the control element toward the second end position; and a flow orifice disposed in the main connection line, wherein the throttle and the orifice are disposed in series in the main connection line, and wherein, as viewed in movement direction, the control element is acted upon on a side confronting the first end position by gas pressure commensurate with a gas pressure upstream of the orifice, and acted upon on a side confronting the second end position by gas pressure commensurate with a gas pressure downstream of the orifice, for controlling the effective area of the control opening in dependence on a difference between the gas pressures upstream and downstream of the orifice.
Depending on the size of the volume flow, different pressures are encountered in flow direction on opposite sides of the orifice, i.e. a pressure differential exists across the orifice. These gas pressures act on opposite sides of the control element to move the control element until the elastic counterforce effectively realizes that the overall force on the control element is zero. This means that the control element is shifted between both end positions until reaching a position in which the overall force acting on the control element has substantially vanished except for slight negligible friction losses. Depending on the movement direction, the effective area of the control element becomes smaller or greater during displacement. Thus, a closed-loop control circuit is established for the volume flow and is controllable proportional to the square root of the difference of the gas pressures on opposite sides of the orifice.
According to another feature of the present invention, the cross bore is a cylindrical throughbore, and the control element is a control piston made of plastic and having a cylindrical configuration that complements the cylindrical configuration of the throughbore. In this way, the control piston can be displaced in the throughbore substantially free of friction.
According to another feature of the present invention, the control element is shifted by the counterforce member to a position which is commensurate with the first end position, when no gas pressure acts upon the control member. Suitably, the control element reaches this position, when the gas pressure differential is zero and the control opening has a maximum effective area, so that the full lifting force is immediately effective in a lifting drive, when a nominal weight is to be lifted.
The counterforce may be formed by an elastic element, e.g. a helical spring.
In order to realize a soft opening of the throttle, the main connection line may include two parallel ring channels which are connectable to one another by the control opening only. The connection of the ring channels may be implemented by providing the control opening through formation of V-shaped grooves in the outer surface area of the control element. The provision of such grooves results in a particularly soft opening of the throttle. Suitably, each of the V-shaped grooves has side surfaces defining a triangle to simplify manufacture while still maintaining a soft opening of the throttle.
An effective orifice area can be implemented in a simple manner by providing an adjustment cylinder having a conically tapered end portion for projection through the orifice so that the effective orifice area can be adjusted through appropriate positioning of the cylinder.
According to another feature of the present invention, the control element may be connected on the side confronting the first end position by a first auxiliary connection line with a portion of the main connection line, disposed directly upstream of the orifice, and connected on the side confronting the second end position by a second auxiliary connection line with a portion of the main connection line, disposed directly downstream of the orifice. Suitably, a reversing valve may be provided for switching connections of the first and second auxiliary connection lines such that the first auxiliary connection line is connected with the portion of the main connection line, disposed directly downstream of the orifice, and the second auxiliary connection line is connected with the portion of the main connection line, disposed directly upstream of the flow orifice, wherein the one of the first and second ports intended for outgoing operating gas is disconnected by the reversing valve from the gas supply and connected for venting.